Identification of an Old Planetary Nebula Around the PG 1159 Star: PG 1520+525

We have surveyed two PG 1159 class stars for the presence of ancient planetary nebulae by direct Hα imaging. While we easily found an 11' diameter nebula around PG 1520+525, no nebula was detected around PG 1424+535. This nebula is the tenth member of the class of planetary nebulae surrounding PG 1159 stars. It now seems extremely likely that planetary nebulae are the direct progenitors of PG 1159 stars and the hydrogen deficient white dwarfs. Nevertheless, not all PG 1159 stars have detectable nebulae around them, probably because small differences in nebular ejection velocities and central star masses have a disproportionately large effect on the detectability of the nebulae. A 25% increase in expansion velocity or a 4% decrease in central star mass reduces the surface brightness by a factor of 2 by the time the star reaches the PG 1159 state. The nebula around PG 1520+525 is limb brightened to the south. While possibly due to an interaction with the ISM, the case is weak because the central star fails to exhibit a high proper motion and because the ISM density is not expected to be sufficient to induce the observed effect at the Galactic height (∼0.6 kpc) of the nebula. If an interaction is present, then the ISM in the vicinity of the nebula has a density ∼0.05 cm-3. We consider arguments that the distance to PG 1520+525 has been overestimated by the model atmosphere analysis method. At a distance of 1.1 kpc, the kinematic age of the nebula is three times the predicted post-AGB lifetime of the exciting star, the nebula is among the very largest known in the Galaxy, and the ∼1.4 Msun nebula is abnormally massive. We find that a distance of 0.8 kpc violates none of the observational constraints by more than 1σ and, therefore, is preferred.